ffMathy · ffMathy · Feb 21, 2020 · Feb 19, 2020 · Feb 19, 2020 · Feb 19, 2020
diff --git a/README.md b/README.md
@@ -140,6 +140,24 @@ console.log(fakeCalculator.divide(10, 5)); //prints 5
 console.log(fakeCalculator.divide(9, 5)); //prints 1338
 ```
 
+## Throwing exceptions
+Exceptions can be thrown on properties or methods. You can add different exceptions for different arguments
+
+```typescript
+import { Substitute, Arg } from '@fluffy-spoon/substitute';
+
+interface Calculator {
+  add(a: number, b: number): number;
+  subtract(a: number, b: number): number;
+  divide(a: number, b: number): number;
+  isEnabled: boolean;
+}
+
+const calculator = Substitute.for<Calculator>();
+calculator.divide(Arg.any(), 0).throws(new Error('Cannot divide by 0'));
+calculator.divide(1, 0); // throws the exception Error: Cannot divide by 0
+```
+
 # Benefits over other mocking libraries
 - Easier-to-understand fluent syntax.
 - No need to cast to `any` in certain places (for instance, when overriding read-only properties) due to the `myProperty.returns(...)` syntax.

diff --git a/package-lock.json b/package-lock.json
diff --git a/spec/throws.spec.ts b/spec/throws.spec.ts
@@ -0,0 +1,42 @@
+import test from 'ava'
+
+import { Substitute, Arg } from '../src/index'
+
+interface Calculator {
+  add(a: number, b: number): number
+  divide(a: number, b: number): number
+  mode: boolean
+  fakeSetting: boolean
+}
+
+test('throws on a method with arguments', t => {
+  const calculator = Substitute.for<Calculator>()
+  calculator.divide(Arg.any(), 0).throws(new Error('Cannot divide by 0'))
+
+  t.throws(() => calculator.divide(1, 0), { instanceOf: Error, message: 'Cannot divide by 0' })
+})
+
+test('throws on a property being called', t => {
+  const calculator = Substitute.for<Calculator>()
+  calculator.mode.throws(new Error('Property not set'))
+
+  t.throws(() => calculator.mode, { instanceOf: Error, message: 'Property not set' })
+})
+
+test('does not throw on methods that do not match arguments', t => {
+  const calculator = Substitute.for<Calculator>()
+  calculator.divide(Arg.any(), 0).throws(new Error('Cannot divide by 0'))
+  calculator.divide(4, 2).returns(2)
+
+  t.is(2, calculator.divide(4, 2))
+  t.throws(() => calculator.divide(1, 0), { instanceOf: Error, message: 'Cannot divide by 0' })
+})
+
+test('can set multiple throws for same method with different arguments', t => {
+  const calculator = Substitute.for<Calculator>()
+  calculator.divide(Arg.any(), 0).throws(new Error('Cannot divide by 0'))
+  calculator.divide(Arg.any(), Arg.is(number => !Number.isInteger(number))).throws(new Error('Only integers supported'))
+
+  t.throws(() => calculator.divide(1, 1.135), { instanceOf: Error, message: 'Only integers supported' })
+  t.throws(() => calculator.divide(1, 0), { instanceOf: Error, message: 'Cannot divide by 0' })
+})
diff --git a/src/Transformations.ts b/src/Transformations.ts
@@ -1,16 +1,17 @@
 import { AllArguments } from "./Arguments";
 
-export type NoArgumentFunctionSubstitute<TReturnType> = 
+export type NoArgumentFunctionSubstitute<TReturnType> =
     (() => (TReturnType & NoArgumentMockObjectMixin<TReturnType>))
 
-export type FunctionSubstitute<TArguments extends any[], TReturnType> = 
-    ((...args: TArguments) => (TReturnType & MockObjectMixin<TArguments, TReturnType>)) & 
+export type FunctionSubstitute<TArguments extends any[], TReturnType> =
+    ((...args: TArguments) => (TReturnType & MockObjectMixin<TArguments, TReturnType>)) &
     ((allArguments: AllArguments) => (TReturnType & MockObjectMixin<TArguments, TReturnType>))
 
 export type PropertySubstitute<TReturnType> = (TReturnType & Partial<NoArgumentMockObjectMixin<TReturnType>>);
 
 type BaseMockObjectMixin<TReturnType> = {
     returns: (...args: TReturnType[]) => void;
+    throws: (exception: any) => void;
 }
 
 type NoArgumentMockObjectMixin<TReturnType> = BaseMockObjectMixin<TReturnType> & {
@@ -30,7 +31,7 @@ export type ObjectSubstitute<T extends Object, K extends Object = T> = ObjectSub
 type TerminatingObject<T> = {
     [P in keyof T]:
     T[P] extends () => infer R ? () => void :
-    T[P] extends (...args: infer F) => infer R ? (...args: F) => void : 
+    T[P] extends (...args: infer F) => infer R ? (...args: F) => void :
     T[P];
 }
 
@@ -43,6 +44,6 @@ type ObjectSubstituteTransformation<T extends Object> = {
 
 type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
 
-export type OmitProxyMethods<T extends any> = Omit<T, 'mimick'|'received'|'didNotReceive'>;
+export type OmitProxyMethods<T extends any> = Omit<T, 'mimick' | 'received' | 'didNotReceive'>;
 
 export type DisabledSubstituteObject<T> = T extends ObjectSubstitute<OmitProxyMethods<infer K>, infer K> ? K : never;
diff --git a/src/Utilities.ts b/src/Utilities.ts
@@ -11,6 +11,9 @@ export enum Type {
     property = 'property'
 }
 
+export const Nothing = Symbol();
+export type Nothing = typeof Nothing
+
 export function stringifyArguments(args: any[]) {
     args = args.map(x => util.inspect(x));
     return args && args.length > 0 ? 'arguments [' + args.join(', ') + ']' : 'no arguments';
@@ -62,16 +65,15 @@ export function areArgumentsEqual(a: any, b: any) {
 
 function deepEqual(a: any, b: any): boolean {
     if (Array.isArray(a)) {
-        if (!Array.isArray(b) || a.length !== b.length)
-            return false;
+        if (!Array.isArray(b) || a.length !== b.length) return false;
         for (let i = 0; i < a.length; i++) {
-            if (!deepEqual(a[i], b[i]))
-                return false;
+            if (!deepEqual(a[i], b[i])) return false;
         }
         return true;
     }
     if (typeof a === 'object' && a !== null && b !== null) {
         if (!(typeof b === 'object')) return false;
+        if (a.constructor !== b.constructor) return false;
         const keys = Object.keys(a);
         if (keys.length !== Object.keys(b).length) return false;
         for (const key in a) {

diff --git a/src/states/FunctionState.ts b/src/states/FunctionState.ts
@@ -3,17 +3,24 @@ import { Context } from "src/Context";
 import { areArgumentArraysEqual, Call, Type } from "../Utilities";
 import { GetPropertyState } from "./GetPropertyState";
 
-const Nothing = Symbol()
-
 interface ReturnMock {
     args: Call
     returnValues: any[] | Symbol // why symbol, what
     returnIndex: 0
 }
+interface MimickMock {
+    args: Call
+    mimickFunction: Function
+}
+interface ThrowMock {
+    args: Call
+    throwFunction: any
+}
 
 export class FunctionState implements ContextState {
     private returns: ReturnMock[];
-    private mimicks: Function|null;
+    private mimicks: MimickMock[];
+    private throws: ThrowMock[];
 
     private _calls: Call[]; // list of lists of arguments this was called with
     private _lastArgs?: Call // bit of a hack
@@ -32,8 +39,9 @@ export class FunctionState implements ContextState {
 
     constructor(private _getPropertyState: GetPropertyState) {
         this.returns = [];
-        this.mimicks = null;
+        this.mimicks = [];
         this._calls = [];
+        this.throws = [];
     }
 
     private getCallCount(args: Call): number {
@@ -51,15 +59,22 @@ export class FunctionState implements ContextState {
             this.property,
             args);
 
-        if(!hasExpectations) {
+        if (!hasExpectations) {
             this._calls.push(args)
         }
 
         if (!hasExpectations) {
-            if(this.mimicks)
-                return this.mimicks.apply(this.mimicks, args);
+            if (this.mimicks.length > 0) {
+                const mimicks = this.mimicks.find(mimick => areArgumentArraysEqual(mimick.args, args))
+                if (mimicks !== void 0) return mimicks.mimickFunction.apply(mimicks.mimickFunction, args);
+            }
+
+            if (this.throws.length > 0) {
+                const possibleThrow = this.throws.find(throws => areArgumentArraysEqual(throws.args, args))
+                if (possibleThrow !== void 0) throw possibleThrow.throwFunction;
+            }
 
-            if(!this.returns.length)
+            if (!this.returns.length)
                 return context.proxy;
             const returns = this.returns.find(r => areArgumentArraysEqual(r.args, args))
 
@@ -85,16 +100,36 @@ export class FunctionState implements ContextState {
         if (property === 'then')
             return void 0;
 
-        if(property === 'mimicks') {
+        if (property === 'mimicks') {
             return (input: Function) => {
-                this.mimicks = input;
+                if (!this._lastArgs) {
+                    throw new Error('Eh, there\'s a bug, no args recorded for this mimicks :/')
+                }
+                this.mimicks.push({
+                    args: this._lastArgs,
+                    mimickFunction: input
+                })
                 this._calls.pop()
 
                 context.state = context.initialState;
             }
         }
 
-        if(property === 'returns') {
+        if (property === 'throws') {
+            return (input: Error | Function) => {
+                if (!this._lastArgs) {
+                    throw new Error('Eh, there\'s a bug, no args recorded for this throw :/')
+                }
+                this.throws.push({
+                    args: this._lastArgs,
+                    throwFunction: input
+                });
+                this._calls.pop();
+                context.state = context.initialState;
+            }
+        }
+
+        if (property === 'returns') {
             return (...returns: any[]) => {
                 if (!this._lastArgs) {
                     throw new Error('Eh, there\'s a bug, no args recorded for this return :/')
@@ -106,7 +141,7 @@ export class FunctionState implements ContextState {
                 })
                 this._calls.pop()
 
-                if(this.callCount === 0) {
+                if (this.callCount === 0) {
                     // var indexOfSelf = this
                     //     ._getPropertyState
                     //     .recordedFunctionStates

diff --git a/src/states/GetPropertyState.ts b/src/states/GetPropertyState.ts
@@ -1,13 +1,12 @@
 import { ContextState, PropertyKey } from "./ContextState";
 import { Context } from "src/Context";
 import { FunctionState } from "./FunctionState";
-import { Type } from "../Utilities";
-
-const Nothing = Symbol();
+import { Type, Nothing } from "../Utilities";
 
 export class GetPropertyState implements ContextState {
-    private returns: any[]|Symbol;
-    private mimicks: Function|null;
+    private returns: any[] | Nothing;
+    private mimicks: Function | Nothing;
+    private throws: any;
 
     private _callCount: number;
     private _functionState?: FunctionState;
@@ -30,7 +29,8 @@ export class GetPropertyState implements ContextState {
 
     constructor(private _property: PropertyKey) {
         this.returns = Nothing;
-        this.mimicks = null;
+        this.mimicks = Nothing;
+        this.throws = Nothing;
         this._callCount = 0;
     }
 
@@ -58,10 +58,10 @@ export class GetPropertyState implements ContextState {
         if (property === 'then')
             return void 0;
 
-        if(this.isFunction)
+        if (this.isFunction)
             return context.proxy;
 
-        if(property === 'mimicks') {
+        if (property === 'mimicks') {
             return (input: Function) => {
                 this.mimicks = input;
                 this._callCount--;
@@ -70,8 +70,8 @@ export class GetPropertyState implements ContextState {
             }
         }
 
-        if(property === 'returns') {
-            if(this.returns !== Nothing)
+        if (property === 'returns') {
+            if (this.returns !== Nothing)
                 throw new Error('The return value for the property ' + this._property.toString() + ' has already been set to ' + this.returns);
 
             return (...returns: any[]) => {
@@ -82,21 +82,33 @@ export class GetPropertyState implements ContextState {
             };
         }
 
-        if(!hasExpectations) {
-            this._callCount++;
-
-            if(this.mimicks)
-                return this.mimicks.apply(this.mimicks);
+        if (property === 'throws') {
+            return (callback: Function) => {
+                this.throws = callback;
+                this._callCount--;
 
-            if(this.returns !== Nothing) {
-                var returnsArray = this.returns as any[];
-                if(returnsArray.length === 1)
-                    return returnsArray[0];
-
-                return returnsArray[this._callCount-1];
+                context.state = context.initialState;
             }
         }
 
+        if (!hasExpectations) {
+                this._callCount++;
+
+                if (this.mimicks !== Nothing)
+                    return this.mimicks.apply(this.mimicks);
+
+                if (this.throws !== Nothing)
+                    throw this.throws
+
+                if (this.returns !== Nothing) {
+                    var returnsArray = this.returns as any[];
+                    if (returnsArray.length === 1)
+                        return returnsArray[0];
+
+                    return returnsArray[this._callCount - 1];
+                }
+            }
+
         context.initialState.assertCallCountMatchesExpectations(
             [[]],  // I'm not sure what this was supposed to mean
             this.callCount,

diff --git a/src/states/SetPropertyState.ts b/src/states/SetPropertyState.ts
@@ -2,8 +2,6 @@ import { ContextState, PropertyKey } from "./ContextState";
 import { Context } from "src/Context";
 import { areArgumentsEqual, Type } from "../Utilities";
 
-const Nothing = Symbol();
-
 export class SetPropertyState implements ContextState {
     private _callCount: number;
     private _arguments: any[];